1. Field of the Invention
The present invention relates in general to the cementing of a prosthesis in a bone cavity and specifically to apparatus for use with a source of bone cement having a movable piston for supplying the cement under pressure to a bone cavity. The attachment enables a user to supply the cement to the bone cavity in controllable and exact small quantities and consequently increase pressure in more precise increments than is possible by use of presently available devices.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Commonly owned U.S. Pat. No. 4,357,716 and copending application Ser. No. 09/169,737 now U.S. Pat. No. 5,951,563, entitled "Sensor System for Flowable Cement", filed on the same date as this application by the same inventor, are both incorporated in their entirety herein by reference. The U.S. patent discloses a system for mounting a femoral stem prosthesis in the femoral canal with the use of a cement. Copending patent application Ser. No. 09/169,737 now U.S. Pat. No. 5,951,563 discloses a sensor for enabling the viscosity of a flowable cement to be detected as it is pressurized into the bone cavity section so as to prevent the flowable cement from passing through the intertrabecular spaces in the bone wall into the blood stream of the patient.
The process disclosed in U.S. Pat. No. 4,357,716 includes sealing the bone cavity section in a substantially fluid-tight relationship, inserting the cement through a seal into the bone cavity under pressure, having an air escape orifice that allows air and body fluids to escape the bone cavity until it is full of cement, plugging the air escape cavity, and holding the pressure on the cement until it sets. Another method of pressurizing bone cement in a bone cavity consists of partially filling the cavity with cement then, using a cannulated plastic or rubber plug at the end of the bone cavity, inserting the nozzle of the cement syringe through the plug, and adding cement and pressure for a period of approximately 20 seconds. Such pressurization is done at approximately 4 minutes from beginning to mix at a time when the cement is of low viscosity. The pressure which is applied is that amount that "feels right". The plug and syringe are then removed and the prosthesis inserted into the cavity.
Yet another method of pressurizing cavities with cement is to use orifices in the prosthesis itself to inject the cement under pressure.
Presently, the major compounds of bone-cement consist of a liquid monomer of methylmethacrylate, a powder, polymethacrylate, and radiopaque barium sulfate. The liquid and powder may be mixed by one of several methods and then deposited into a gun or syringe having a long nozzle. The cement is injected in a retrograde fashion, i.e. from the bottom up to near the top of the femoral canal. The initial injection of cement is somewhat timely. It should begin when the mixture is in the early doughy state. The cement should be of such viscosity that the femoral canal can be satisfactorily filled without entrapment, or at least with minimal entrapment, 1of air pockets. At the same time, the cement should be of such viscosity that it will partially fill the intertrabecular spaces but not flow from the intertrabecular spaces into the veins beyond the cortical bone. Usually the mixture changes from a liquid state to an early doughy state at about three minutes from beginning to mix the liquid monomer and the powder polymer.
The change from the stage when the mixture is a liquid to a slightly doughy state usually occurs in a matter of a few seconds. That period of time varies somewhat with different brands of cement. Other factors also influence the speed of this reaction or change. A variation of six degrees temperature markedly alters the speed of the reaction. A higher temperature increases the speed. Therefore, the room temperature, the overhead operating room lights, the temperature of the vial and monomer, the temperature of the package powder and the mixing bowl and the stirring ladle all influence the speed of the reaction.
Further, the method and seed of mixing affect the speed of the reaction. The greater the amount of oxygen that is mixed with the cement the more the speed of the reaction will be decreased. Most mixing is done in a container with a vacuum attached to help remove ambient vapor from the operating room. The vacuum pulls air into the mixing bowl and so the length of time that the vacuum is attached and the amount of suction influences the amount of oxygen that will pass over the mixture as well as the degree of evaporation. Further, the monomer-polymer ratio may also have been altered which affects the speed of the reaction.
The fact is, it is extremely important for the physician to monitor the viscosity of the cement so that, when it is still in a highly liquid state, it will not be forced under pressure through the intertrabecular spaces to the veins lying beyond the cortical wall.
A method of pressurizing cement in the femoral canal or other bone cavity is that of inserting the prosthesis through a sealing device and inserting it in the femoral canal and attaching the sealing device to the canal or bone cavity in a substantially fluid-tight relationship and injecting cement through orifices in the sealing device. This is clearly shown in commonly owned U.S. Pat. No. 4,357,716. The sealing device in U.S. Pat. No. 4,357,716 includes an orifice for venting the canal so that air and other bodily fluids in the femoral canal can be displaced by the cement and escape through the orifice. However, when cement begins to exude from the orifice, it is then closed so that cement can no longer exude and the pressure is increased to compress the cement in an effort to remove and compress air bubbles and to cause the cement to fill the intertrabecular spaces and surround the prosthesis in a binding relationship. The hole from which the cement can exude is a one-quarter inch hole which, of course, allows the fluid products to easily flow therethrough.
Thus, when the orifice is sealed and pressure is applied, if the cement does not have sufficient viscosity, the pressure applied may force it through the intertrabecular spaces and Haversian canals and into the veins on the other side of the cortical wall.
If cement does flow into the veins on the other side of the cortical wall, there may be extremely adverse effects on the patient's health.
Copending application Ser. No. 09/169,737 now U.S. Pat. No. 5,951,563 discloses novel apparatus and methods to monitor the viscosity of the cement to which pressure is being applied so that pressure of greater amounts than are needed or desired will not be applied when the viscosity is low and so that a maximal amount of pressure can be applied when the viscosity has increased to the point it cannot flow through the intertrabecular spaces and into the circulation beyond the bone.
It is also extremely important that the user be able to selectively increase the pressure at which the cement is introduced into the bone cavity as the viscosity increases, to assure that the cement is not inadvertently forced through the intertrabecular spaces of the bone.
Thus, it would be desirable to have a more exact and controlled way of incrementally increasing the pressure applied to the cement as the viscosity of the cement increases.